The present invention generally relates to power system protection, and more specifically, to a system, apparatus and method for compensating the sensitivity of a sequence element in a line current differential relay in a power system.
Electric power systems are designed to generate, transmit and distribute electrical energy to loads. In order to accomplish this, power systems generally include a variety of power system elements such as electrical generators, electrical motors, power transformers, power transmission lines, buses and capacitors, to name a few. As a result, power systems typically include protective devices and associated procedures to protect the power system elements from abnormal conditions such as electrical short circuits, overloads, frequency excursions, voltage fluctuations, and the like.
A protective device and associated procedure acts to isolate some power system element(s) from the remainder of the power system upon detection of the abnormal condition or a fault in, or related to, the power system element(s). Logically grouped zones of protection, or protection zones utilizing the protective devices and procedures, are established to efficiently manage faults or other abnormal conditions occurring in the power system elements.
In general, protection zones may be classified into six types including: (1) generators and generator-transformer elements (2) transformers, (3) buses, (4) lines (transmission, sub-transmission and distribution), (5) utilization equipment (motors, static loads), and (6) capacitor or reactor banks. As a result, a variety of protective devices are required. Such protective devices may include different types of protective relays, surge protectors, arc gaps and associated circuit breakers and reclosers.
Although the fundamentals of power system protection are similar, each of the six types of protection zones uses protective devices that are based on the characteristics of the power system elements in that category. More specifically, different protective relays utilizing a variety of protective schemes (e.g., differential current comparisons, magnitude comparisons, frequency sensing), are required to protect the various power system elements. For example, a line current differential relay, having electrical connections to the transmission line via current transformers (designed to step-down the primary current to a magnitude suitable for use by the line current differential relay), is designed to monitor current flowing in a transmission line by measuring the current flowing into and out of terminal points of the transmission line, and calculating inter alia, the sum of all measured current, or the operate current. As is known, when the transmission line is operating under normal conditions, the sum of all of the (primary) currents entering the line is about zero (Kirchhoff's current law). If the transmission line has a short circuit or is faulted, the corresponding relay operate current will be substantially different from zero, indicating that there is some impermissible path through which a current is flowing. If the operate current exceeds some threshold, or pickup current, and some differential inequality is satisfied, the line current differential relay issues a trip signal to an associated power circuit breaker(s) causing it to open and isolate the faulted bus from the remainder of the power system.
For example, in the case of multi-terminal line using the percentage differential principle, we would have the next criteria for operation . . . ,Ioperate>k·IrestraintandIoperate>Ipuwhere Ioperate=|Ī1+Ī2+Ī3+ . . . Īn or the phasor sum of currents flowing in the protection zone, and Irestraint=(|Ī1|+|Ī2|+|Ī3|+ . . . |Īn|)/p or the sum of the absolute value of the current phasors representing the current flowing through the protection zone, where k=differential percentage, Ipu=minimum pickup current value, and p=number of terminals. Thus, the line current differential relay requires that the operate current exceeds a minimum pickup value and some percentage of the total current flowing through the protection zone before the line current differential relay may issue a breaker tripping signal.
Typical protection of a transmission line is generally performed using two line current differential relays coupled to the transmission line via current transformers as described above, with each relay located at different extremities of the line. Such a configuration requires that each differential relay communicates its current and voltage measurement data to the other via a communication channel (e.g., microwave channel, telephone grade channel, fiber optics, etc.).
Providing line current differential protection for a transmission line generally includes incorporation of both phase elements and sequence elements in the line current differential relay; that is, incorporation of an A-phase element, a B-phase element and a C-phase element, and a possibly a zero-sequence or ground element and a negative-sequence element. Depending upon the type of fault detected in the transmission line, one or more of the phase and/or sequence elements will operate to cause the line current differential relay to assert and subsequently issue a trip signal to an associated power circuit breaker(s). For example, in the case of an A-phase-to-ground fault, the A-phase element, the zero-sequence element, and the negative-sequence element may operate.
As is known, however, some or all of the A-phase element, the zero-sequence element, and/or the negative-sequence element may fail to operate properly when certain conditions exist. For example, if the fault resistance, Rf of the A-phase-to-ground fault is above the resistive limit of sensitivity, the A-phase element may fail to properly detect the fault. In other words, under certain circumstances (such as an overloaded line, a highly resistive fault, a pole-open condition), an unbalanced network condition, or a cross-country fault condition, some or all of the A-, B- and/or C-phase current elements (“phase elements” ), and the zero-sequence phase element and/or the negative-sequence phase element (“sequence elements” ) may “lose their sensitivity to detect a fault condition”. Accordingly, a fault may go undetected by some or all of the three phase and/or the two sequence elements of the line current differential relay(s), possibly resulting in removal from operation a larger portion of the power system network by the back-up protection.